1. Field of the invention
The present invention relates generally to a piezoelectric motor and in particular to a piezoelectric motor with one or more actuators having a linear contact with the rotor which can be used generally as a high performance replacement for small conventional electric motors used in computer equipment, robotics, manufacturing equipment, aerospace, automobiles, toys, etc. The motor of the present invention can be used particularly effectively in various disk drive devices such as for magnetic and optical data storage equipment, especially when small size and weight are required for compactness.
2. Description of the prior art
In order to develop adequate torque, to control/reduce revolution speed and to achieve desired positioning accuracy, many conventional electric motors rated under 10 watts operate at higher revolution speeds and use gearing mechanisms to reduce RPMs. Small electric motors typically have efficiency ratings in the general area of 50% to 75%. With miniature motors under 2 watts, efficiency declines below 50%. Also, the parasitic losses from the gearing mechanism become proportionately more significant. Power available at the shaft may be generated at an efficiency of only 10% to 25%.
Also, as the diameter of a motor becomes very small, its torque generating capability declines because torque is the product of force times the distance it is applied from the shaft axis. This limits the ability to reduce size (diameter) and weight. Start-stop dynamics are poor with electric motors because of the inertia of the rotor. Additionally, the combined electric motor and gearing mechanism includes many, many parts and its manufacture becomes more difficult and complex as they become very small.
Piezoelectric ultrasonic motors were invented over 30 years ago. Their basic operating mechanism is that when voltage is applied across a piezoelectric material, for example a quartz crystal, it causes the crystal to change its shape, or bend. When voltage of an opposite sign is applied, the crystal bends the opposite way. If alternating current is applied to the crystal, it vibrates at the frequency of the alternating current. Piezoelectric motors operate by placing a vibrating piezoelectric element in contact with another free moving element, inducing its rotation or movement. Controlling the contact mechanics between these two elements has always been a large part of developmental efforts for piezoelectric motors.
Piezoelectric ultrasonic motors have many advantages over small conventional electromagnetic motors, especially those rated less than two watts. They are variable speed motors with high torque at low revolution speeds, allowing for the elimination of gear mechanisms. They are quiet and more energy efficient. Their power density does not decline with size so they gain an increasing comparative advantage as the size of the motor declines. They have greater positioning accuracy-they are like stepper motors with each step being less than 10 microns. Response time is up to ten times faster and stability is better. The electronic controls can be more simple. When the motor is turned off, it resists shaft movement. They are much less sensitive to extreme ambient temperature environments. Lastly, they produce no environmental electromagnetic fields.
Two broad categories of piezoelectric motors can be identified analyzing the prior art:
motors with a "surface contact" in which the area between the piezoelectric driving element and the rotor or other driven structure is wide so that it is the surface of the vibrating actuator itself that provides a firm contact with and ultimately propels the rotor, and PA1 motors with a "linear contact" in which the area between the piezoelectric actuator and the rotor has a small width of about 0.5 mm or less.
Surface contact piezoelectric motors are quite reliable, and typically utilize a "traveling wave" principle, containing at least one piezoelectric disc divided into a multitude of driving piezoelectric sectors that bend in a coordinated fashion to produce a wave which induces a contacting surface to rotate or move. Linear contact motors, also known as "standing wave" motors, typically have a vibrating element creating an elliptical orbit at its working end, which pushes the driven member by frictional contact. Given the small area of contact and the abrasive nature of the interaction, the useful life of these motors is small, ranging from 1to 3,000 hours.
Surface wear is much less of a problem for traveling wave motors due to their larger contact area. Most piezoelectric motor developmental efforts have been on the traveling wave type because they are capable of much higher torque levels and have a much longer useful life. Compared to the standing wave type, they are much more expensive and complex, require more complicated electronics, are less energy efficient and have less positioning accuracy.
Even though they have many performance advantages, piezoelectric motors have very few commercial applications. This is primarily due to cost. Traveling wave piezoelectric motors are very expensive. Standing wave motors are an order of magnitude less in cost, but the expected useful life of current designs is not acceptable. For all piezoelectric motors, manufacturing economies of scale have never been achieved due to the limited volume of their small, often custom, applications base.
Piezoelectric motors are now used in industry in applications when small size, quiet operation, high performance and high efficiency are required. One limitation of such devices is their limited torque. In order to increase the torque and widen the application of the motor it is known to utilize more than one actuator driving the same rotor.
An example of a traveling wave piezoelectric motor can be found in the U.S. Pat. No. 5,532,541 by Fujishima. An ultrasonic motor of this invention has a plurality of so called Langevin vibrators arranged in a ring. Each of the vibrators has a first and a second polarized region. When alternating voltage is supplied to the vibrators, the edges of the metal terminals move in an elliptical motion, which drives the rotor.
This and other similar devices of that type have intrinsically limited energy efficiency due to the fact that the bias force needed to maintain a good friction engagement between the stator and the rotor over a large contact area have to be substantial. In addition to the energy losses, shortened operational life results from these significant bias forces which cause deterioration of the contact surface.
Linear contact piezoelectric motors represent another type of an ultrasound motor. Most known devices of this type contain a "standing wave" vibrational element in which the working edge of a piezoelectric member moves in a cyclical pattern and is placed in direct contact with the rotor. Example of such device can be found in U.S. Pat. No. 3,211,931 by Tehon which describes a motor having a magnetostrictive or electrostrictive element transmitting torque through a series of mechanical couplers with an edge contact to a rotating shaft.
A number of designs for such motors is described by Vishnevsky et al in U.S. Pat. Nos. 4,019,073; 4,400,641; 4,453,103; and 4,959,580 all of which are incorporated herein by reference. In addition to depicting the general principles of a linear contact motor, these patents propose a number of designs with multiple actuators. According to the invention, in these designs a single rotor is typically activated by a number of radially placed piezoelectric vibrators. Such designs are generally useful for the purposes of increasing the motor torque while maintaining the compact nature of the motor. Still, among fundamental disadvantages of the piezoelectric motors of the prior art and specifically the designs depicted in these patents, one can point to the limited operational life which is a direct consequence of the nature of actuator/rotor interaction. When two hard surfaces interact according to this and other prior art designs, the rotational energy is transferred predominantly by friction. To avoid slippage of the rotor relative to the actuator and to transmit higher torques, the bias forces have to be quite high once again which leads not only to a reduction of energy efficiency but also to eventual disintegration of the motor elements which reduces the limits of the operational life. As our studies have demonstrated, in a typical case the operational life of this motor is limited to only 1 to 3 thousand hours or less which renders the practical applicability of the motor problematic. There is a need therefore for a piezoelectric motor with a linear contact having higher reliability and extended operational life of at least 5 thousand hours or more.
As was pointed out in our Russian Patent No. 524,222, piezoelectric motors can be used advantageously in the area of disk drives and turntable devices. For the purposes of this specification, the term "disk" includes a variety of electronic data storage devices such as optical and magnetic disks, computer hard drives, CD-ROM and other compact disks, media storage devices such as sound records etc. The advantage of piezoelectric motors is in their quiet operation, improved performance, small size, attractive shape and diameter/height ratio. In general, the use of piezoelectric motors in this area of technology is described for example in U.S. Pat. Nos. 5,805,540 by Kitai, 5,140,566 by Kang, and 5,600,613 by Matsumoto.
In spite of the known advantages of the piezoelectric motors, the motors of the prior art did not find widespread use in the disk drive area because of the above mentioned limitations, mainly the low energy efficiency (which depletes the batteries for some of the portable disk drive devices such as in a lap top computer) and the limited operational time. The need exists therefore for an improved low power disk drive device of the linear contact type with extended operational life. Such device would allow the utilization of all intrinsic advantages of this type of motors and therefore allow for significant improvements in performance through the reduction in weight and size of computers and other general devices requiring a disk drive function.
Other applications requiring high performance electric motors (either motors that use a rotor or those referred to as linear motors) of small size would find attractive the higher performance characteristics of piezoelectric motors if their cost was lower and more competitive with conventional motors.